Ground water for the preparation of water for industrial and/or domestic use distributed by public utilities is increasingly contaminated with unhealthy or otherwise undesirable materials due to the growing strain the soil is subjected to, e.g. by means of over-fertilization or extensive use of herbicides, pesticides, insecticides etc. As a result, the preparation of potable domestic and/or industrial water is increasingly difficult and requires the use of additional purifying assemblies. These assemblies are known per se and operate on the principle of reverse osmosis. According to this principle the untreated or otherwise madeup water deriving from ground water is fed to the reverse osmosis unit of the assembly. One part of the water permeates the membrane of the unit and undesirable impurities are removed. This part of the water subsequently leaves the unit and is available to be used as potable domestic water. The remaining part not having permeated the membrane also leaves the unit to be used as industrial and/or domestic water for appropriate purposes. Such assemblies are designed for industrial large-scale use and result in considerable construction requirements with corresponding production costs. This is predominantly due to the two pumps being installed separately and each pump being provided with its own drive motor. Thus such assemblies are unsuitable for private use.